EP0217437A1 - Process and apparatus for eliminating nitrogen oxides from a gaseous exhaust stream in the humid way - Google Patents

Process and apparatus for eliminating nitrogen oxides from a gaseous exhaust stream in the humid way Download PDF

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EP0217437A1
EP0217437A1 EP86201521A EP86201521A EP0217437A1 EP 0217437 A1 EP0217437 A1 EP 0217437A1 EP 86201521 A EP86201521 A EP 86201521A EP 86201521 A EP86201521 A EP 86201521A EP 0217437 A1 EP0217437 A1 EP 0217437A1
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washing liquid
alkaline
tank
acid
liquid
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German (de)
French (fr)
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Jacques-Pierre Dyens
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Bekaert NV SA
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/46—Removing components of defined structure
    • B01D53/54—Nitrogen compounds
    • B01D53/56—Nitrogen oxides

Definitions

  • the present invention relates to the purification of gaseous effluents containing nitrogen oxides.
  • gaseous effluents containing nitrogen oxides In particular it relates to the removal of these nitrogen oxides from the effluent by the wet route.
  • This type of effluent is recovered, for example in the manufacture of nitric acid, during nitriding reactions, in the fertilizer industry, in the field of hydrometallurgy and in the field of surface treatment (pickling steels, aluminum, titanium, passivation baths, metallization, etc.).
  • Oxides of nitrogen gas are produced in many processes for the manufacture or treatment of products or materials in which chemical reactions take place. They are considered to be important air pollutants. Generally, a mixture of NO and N02 is encountered, characterized by more or less abundant red vapors. These two chemicals can recombine to give other nitrogen oxides depending on the temperatures.
  • the methods for recovering nitric acid from polluting sources, containing nitrogen oxides generally consume considerable amounts of energy and / or catalysts, and are therefore expensive. Therefore they can only be used economically when the nitrous vapors are concentrated and are not mixed with certain other chemical compounds. Often it is therefore illusory in practice to want to recover nitric acid from effluents. The only solution which then remains to control atmospheric pollution by nitrogen oxides requires a treatment for the purification of effluents without recovery of nitric acid.
  • the object of the invention is therefore to provide a system, i.e. a method and a device for eliminating inexpensive and simple nitrogen oxides of gaseous effluents, more particularly NO 2 .
  • the invention thus makes it possible to control in a simple, economical and inexpensive manner compared to the usual techniques the atmospheric pollution caused by gaseous effluents containing nitrogen oxides.
  • the process for the treatment of gaseous effluents saturated or not with humidity, containing nitrogen oxides is characterized in that it comprises a first operation in which a very large part of the nitrogen oxides (in particular NO 2 ) is transformed during washing in an acid medium and thanks to the addition of urea at a temperature between 30 ° and 60 ° C, in non-aggressive products depending on the reactions: If the gaseous effluents are dry at the start, acid washing can allow saturation of the gaseous effluents with humidity.
  • the gaseous effluents from the first operation are subjected to washing with an alkaline solution in order to neutralize the acids entrained by them at the outlet of the first operation.
  • the consumption of alkaline liquid is generally lower than the first operation and effective.
  • these liquids can be mixed, eg at the end of the operation.
  • the pH of the mixture is then adjusted to the legally permissible value.
  • an oxidizing agent eg hydrogen peroxide in order to transform the nitrites, which have possibly formed in the second operation, in nitrates.
  • the control of the redox potential can then be followed by means of an rH-meter.
  • the method according to the invention can be applied continuously or batchwise.
  • the invention also relates to a device for removing nitrogen oxides from a gaseous effluent.
  • this device is suitable and useful for implementing the method described above.
  • the nitrogen oxides elimination system and its mode of operation will now be described with reference to the appended drawing which represents an embodiment of the invention. Additional characteristics and advantages of the invention will emerge from the description of this embodiment, and of its operation.
  • the device in fact comprises a first washing device 9 provided with an inlet 1 for the gaseous effluent to be treated and means 4 for supplying and distributing 13 an acidic washing liquid containing urea.
  • the washing liquid after having been mls in contact with the gaseous effluent is evacuated from the washer 9 and collected in a tank 2 while the treated gas is evacuated from the washer 9 by a pipe 19.
  • this pipe 19 d the evacuation of the treated gas is connected to an inlet 20 of a second washing apparatus 10 which comprises means 5 for supplying and distributing 28 of an alkaline washing liquid.
  • the purified gas is evacuated through the outlet 3 while the washing liquid is transferred from the apparatus 10 to the collection and regeneration tank 8.
  • the tank 8 is generally equipped with means for conditioning the composition and the temperature of the alkaline bath, in particular an inlet 25 for the addition of water and an inlet 26 for the addition of an alkaline solution, as well as a stirring system 23, a meter pH 15 to monitor the consumption of alkaline product and possibly a heat exchanger 27 to avoid the formation of gels in the bath 8.
  • the tank 8 is connected to the means 5 for supplying the alkaline solution by means of a recirculation pump 24. Valves 32 are arranged to adjust the flow rate of the washing liquid if necessary.
  • the gaseous effluent 1 which enters the washer 9 can be brought into contact with the acid solution in co- or counter-current.
  • a packing section 16 of any type known per se and comprising for example a bed , Berl, Raschig, Pall or other rings as filling material.
  • columns of the spray type, trays, domes or any other contactor can be used, such as a liquid jet ejector, etc.
  • the separator 17 can comprise, for example, lamellar separators or tissue.
  • the acid solution containing the urea is brought to a temperature of 30 ° to 60 ° C by a line 4 to distribution means 13 and sprayed, sprinkled or poured by overflow or by any other means onto the gas so as to ensure a good contact which produces a very intense absorption of nitrous vapors.
  • the acid generally has a relatively high concentration of between 5 and 15% by weight, eg between 5 and 10% by weight and can comprise either an organic acid or nitric, hydrochloric or other acid provided let the pH be between 0 and 1.5, eg between 0.5 and 1. 50 to 500 g / l of urea are added to this solution, either in solid form or in liquid form.
  • the acid washing liquid is circulated and in fact recirculated by means of a pump 14 from a conditioning tank 2.
  • the addition of urea can be done by the inlet 6 and that of the water by the 'entry 7.
  • the gas having passed the first washing operation may still contain very fine acid drops which must be neutralized and eliminated before the gaseous effluent escapes into the air.
  • the gas stream leaving the washer 9 is then transferred via line 19 to the inlet 20 of a second washing and neutralization device 10 which can be produced and operate on the same principle as the first washer 9.
  • Contact must therefore be carried out here between the gaseous effluent and the washing liquid 5 which is an alkaline solution whose pH is between 11 and 14, preferably between 12 and 14.
  • the alkaline agent can be sodium hydroxide . For a pH of 13 to 14, the concentration rises from 5 X to 15% by weight.
  • the gas is brought into contact in the washer 10 in co- or counter-current with the alkaline liquid which is brought by line 5 and distributed by distribution means (spraying, sprinkling, spillage, jet of liquid or other).
  • the gas / liquid contact can also be intensified by passing the gas through a packing section 29 arranged in the washer 10. This packing can then comprise a bed of filling rings or any other means allowing a considerable increase in the surface. of gas / liquid contact as described above for section 16.
  • the washed gas then preferably passes through a droplet separator 30, eg a flap or tissue separator before being emitted to the atmosphere via outlet 3. It should be noted that the alkaline washing is not only used to neutralize acid residues, but also to remove above all nitrogen oxide residues still present in the gas entering the nozzle. entry 20.
  • a gaseous effluent containing 120 ppm of nitrogen oxides was treated with the acid and alkaline cascade washing system as described above.
  • the flow rate of the gas stream 1 at the inlet of the washer 9 was 8000 m 3 / h.
  • the initial acid / urea solution had a pH of 0.75 and contained 300 g / l of urea. Its temperature at the place of spraying 13 was 30 ° C.
  • the circulation rate of the acid / urea solution was 35,000 l / h.
  • the bed of filling rings 16 allowed a residence time of 2.5 to 3 sec.
  • the concentration of nitrogen oxides had already fallen to 18 ppm, which corresponds to a yield of 85%.
  • the alkaline washer 10 included a bed 29 of filling rings identical to bed 16.
  • the sodium hydroxide solution had a pH of 13 and it was delivered by the sprayer 28 at 35,000 l / h.
  • the concentration of nitrogen oxides had further decreased to the level of 7 ppm.
  • the efficiency of the alkaline washing therefore amounts to approximately 60% and the total yield of the overall system amounts to 95%.
  • the gaseous effluent to be purified was charged with 6000 to 7000 ppm of nitrogen oxides. It was treated under the same conditions as that of Example 1. At the outlet 19 of the acid washing tower, the concentration had decreased to 900-1000 ppm, which again indicates a yield of approximately 85%. At the outlet of the alkaline washer 10, the concentration of nitrogen oxides had dropped to 500 ppm, which amounts to a yield of approximately 47%. The overall efficiency here therefore amounts to around 92%.

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Abstract

Process comprising an operation of acidic washing containing urea in a scrubber (9) followed by an alkaline washing operation in a scrubber (10) with a view to removing these nitrogen oxides from the effluent. The residual liquids are collected and recirculated from vessels (2, or 8) and can be mixed in a receptacle (18) at the end of the operation with a view to being neutralised. <IMAGE>

Description

La présente invention a trait à l'épuration des effluents gazeux contenant des oxydes d'azote. En particulier elle se rapporte à l'élimination de ces oxydes d'azote de l'effluent par voie humide. On récupère ce type d'effluents p.ex. dans la fabrication d'acide nitrique, au cours des réactions de nitruration, dans l'industrie des engrais, dans le domaine de l'hydrométallurgie et dans le domaine du traitement de surfaces (décapage des aciers, de l'aluminium, du titane, bains de passivation, de métallisation, etc.).The present invention relates to the purification of gaseous effluents containing nitrogen oxides. In particular it relates to the removal of these nitrogen oxides from the effluent by the wet route. This type of effluent is recovered, for example in the manufacture of nitric acid, during nitriding reactions, in the fertilizer industry, in the field of hydrometallurgy and in the field of surface treatment (pickling steels, aluminum, titanium, passivation baths, metallization, etc.).

Des oxydes d'azote gazeux sont produits dans de nombreux procédés de fabrication ou de traitement de produits ou de matériaux où interviennent des réactions chimiques. Ils sont considérés comme des polluants importants de l'atmosphère. Généralement, on rencontre un mélange de NO et N02 se caractérisant par des vapeurs rousses plus ou moins abondantes. Ces deux corps chimiques peuvent se recombiner pour donner d'autres oxydes d'azote en fonction des températures. Les méthodes permettant de récupérer de l'acide nitrique à partir des sources polluantes, contenant des oxydes d'azote, sont généralement consommatrices de quantités considérables d'énergie et/ou de catalyseurs, donc onéreuses. Dès lors ils ne peuvent être utilisés économiquement que lorsque les vapeurs nitreuses sont concentrées et ne sont pas mêlées à certains autres composés chimiques. Souvent il est donc illusoire dans la pratique de vouloir récupérer de l'acide nitrique des effluents. La seule solution qui reste alors pour contrôler la pollution atmosphérique par des oxydes d'azote impose un traitement d'épuration des effluents sans récupération d'acide nitrique.Oxides of nitrogen gas are produced in many processes for the manufacture or treatment of products or materials in which chemical reactions take place. They are considered to be important air pollutants. Generally, a mixture of NO and N02 is encountered, characterized by more or less abundant red vapors. These two chemicals can recombine to give other nitrogen oxides depending on the temperatures. The methods for recovering nitric acid from polluting sources, containing nitrogen oxides, generally consume considerable amounts of energy and / or catalysts, and are therefore expensive. Therefore they can only be used economically when the nitrous vapors are concentrated and are not mixed with certain other chemical compounds. Often it is therefore illusory in practice to want to recover nitric acid from effluents. The only solution which then remains to control atmospheric pollution by nitrogen oxides requires a treatment for the purification of effluents without recovery of nitric acid.

L'objet de l'invention est donc de fournir un système, c.à.d. un procédé et un dispositif permettant l'élimination peu onéreuse et simple des oxydes d'azote d'effluents gazeux, plus particulièrement des NO2. L'invention permet ainsi de contrôler d'une façon simple, économique et peu onéreuse comparée aux techniques habituelles la pollution atmosphérique provoquée par des effluents gazeux contenant des oxydes d'azote.The object of the invention is therefore to provide a system, i.e. a method and a device for eliminating inexpensive and simple nitrogen oxides of gaseous effluents, more particularly NO 2 . The invention thus makes it possible to control in a simple, economical and inexpensive manner compared to the usual techniques the atmospheric pollution caused by gaseous effluents containing nitrogen oxides.

Le procédé de traitement des effluents gazeux saturés ou non en humidité, contenant les oxydes d'azote est caractérisé en ce qu'il comprend une première opération dans laquelle une très grande partie des oxydes de l'azote (en particulier NO2) est transformée lors d'un lavage en milieu acide et grâce à l'adjonction d'urée à une température comprise entre 30° et 60°C, en produits non agressifs suivant les réactions:

Figure imgb0001
Figure imgb0002
Figure imgb0003
Si au départ, les effluents gazeux sont secs, le lavage acide peut permettre la saturation des effluents gazeux en humidité.The process for the treatment of gaseous effluents saturated or not with humidity, containing nitrogen oxides is characterized in that it comprises a first operation in which a very large part of the nitrogen oxides (in particular NO 2 ) is transformed during washing in an acid medium and thanks to the addition of urea at a temperature between 30 ° and 60 ° C, in non-aggressive products depending on the reactions:
Figure imgb0001
Figure imgb0002
Figure imgb0003
 If the gaseous effluents are dry at the start, acid washing can allow saturation of the gaseous effluents with humidity.

Dans une seconde opération, les effluents gazeux provenant de la première opération sont soumis à un lavage à l'aide d'une solution alcaline afin de neutraliser les acides entraînés par eux à la sortie de la première opération. La consommation de liquide alcalin est en général d'autant plus faible que la première opération et efficace.In a second operation, the gaseous effluents from the first operation are subjected to washing with an alkaline solution in order to neutralize the acids entrained by them at the outlet of the first operation. The consumption of alkaline liquid is generally lower than the first operation and effective.

Afin de résoudre en même temps le problème de neutralisation des liquides de lavage évacués respectivement de la première et de la deuxième opération, ces liquides peuvent être mélangés, p.ex. en fin d'opération. Le pH du mélange est alors ajusté à la valeur légalement admissible. Si nécessaire on ajoute un agent oxydant, p.ex. de l'eau oxygénée afin de transformer les nitrites, qui se sont éventuellement formées dans la seconde opération, en nitrates. Le contrôle du potentiel d'oxydo-réduction peut être suivi alors au moyen d'un rH-mètre. Le procédé selon l'invention peut être appliqué en continu ou en discontinu.In order to solve at the same time the problem of neutralization of the washing liquids discharged respectively from the first and from the second operation, these liquids can be mixed, eg at the end of the operation. The pH of the mixture is then adjusted to the legally permissible value. If necessary an oxidizing agent is added, eg hydrogen peroxide in order to transform the nitrites, which have possibly formed in the second operation, in nitrates. The control of the redox potential can then be followed by means of an rH-meter. The method according to the invention can be applied continuously or batchwise.

L'invention se rapporte aussi à un dispositif d'élimination des oxydes d'azote d'un effluent gazeux. En particulier ce dispositif est apte et utile pour la mise-en-oeuvre du procédé décrit ci-dessus. Le système d'élimination des oxydes d'azote et son mode d'opération sera décrit maintenant en regard du dessin annexé qui représente une forme d'exécution de l'invention. Des caractéristiques et avantages supplémentaires de l'invention sortiront de la description de cette forme d'exécution, et de son fonctionnement.The invention also relates to a device for removing nitrogen oxides from a gaseous effluent. In particular, this device is suitable and useful for implementing the method described above. The nitrogen oxides elimination system and its mode of operation will now be described with reference to the appended drawing which represents an embodiment of the invention. Additional characteristics and advantages of the invention will emerge from the description of this embodiment, and of its operation.

Le dispositif comprend en fait un premier appareil laveur 9 pourvu d'une entrée 1 pour l'effluent gazeux à traiter et des moyens d'alimentation 4 et de distribution 13 d'un liquide de lavage acide contenant de l'urée. Le liquide de lavage, après avoir été mls en contact avec l'effluent gazeux est évacué du laveur 9 et collecté dans une cuve 2 tandis que le gaz traité est évacué du laveur 9 par une conduite 19. A son tour, cette conduite 19 d'évacuation du gaz traité est reliée à une entrée 20 d'un second appareil laveur 10 qui comprend un moyen d'alimentation 5 et de distribution 28 d'un liquide de lavage alcalin. Après la mise en contact de la phase liquide alcaline et de la phase gazeuse, le gaz épuré est évacué à travers la sortie 3 tandis que le liquide de lavage est transféré de l'appareil 10 à la cuve 8 de collection et de régénération. La cuve 8 est en général équipée de moyens de conditionnement de la composition et de la température du bain alcalin, notamment d'une entrée 25 pour l'adjonction d'eau et d'une entrée 26 pour l'adjonction d'une solution alcaline, ainsi que d'une système d'agitation 23, d'un mètre pH 15 pour surveiller la consommation de produit alcalin et éventuellement d'un échangeur de chaleur 27 afin d'éviter la formation de gels dans le bain 8. Enfin la cuve 8 est reliée aux moyens 5 d'alimentation de la solution alcaline par l'intermédiaire d'une pompe de recirculation 24. Des robinets 32 sont aménagés pour régler à besoin le débit du liquide de lavage.The device in fact comprises a first washing device 9 provided with an inlet 1 for the gaseous effluent to be treated and means 4 for supplying and distributing 13 an acidic washing liquid containing urea. The washing liquid, after having been mls in contact with the gaseous effluent is evacuated from the washer 9 and collected in a tank 2 while the treated gas is evacuated from the washer 9 by a pipe 19. In turn, this pipe 19 d the evacuation of the treated gas is connected to an inlet 20 of a second washing apparatus 10 which comprises means 5 for supplying and distributing 28 of an alkaline washing liquid. After the alkaline liquid phase and the gaseous phase are brought into contact, the purified gas is evacuated through the outlet 3 while the washing liquid is transferred from the apparatus 10 to the collection and regeneration tank 8. The tank 8 is generally equipped with means for conditioning the composition and the temperature of the alkaline bath, in particular an inlet 25 for the addition of water and an inlet 26 for the addition of an alkaline solution, as well as a stirring system 23, a meter pH 15 to monitor the consumption of alkaline product and possibly a heat exchanger 27 to avoid the formation of gels in the bath 8. Finally, the tank 8 is connected to the means 5 for supplying the alkaline solution by means of a recirculation pump 24. Valves 32 are arranged to adjust the flow rate of the washing liquid if necessary.

L'effluent gazeux 1 qui entre dans le laveur 9 peut être mis en contact avec la solution acide à co- ou contre-courant. Afin d'intensifier ou d'activer le contact et l'échange entre la phase liquide et gazeuse on peut aménager dans la colonne de lavage 9 une section de garnissage 16 d'un type quelconque connu en soi et comprenant p.ex. un lit, d'anneaux Berl, Raschig, Pall ou autre comme corps de remplissage. Alternativement, on peut utiliser des colonnes du type à pulvérisation, à plateaux, à calottes ou tout autre contacteur, comme éjecteur à jet liquide etc. Après que le gaz a traversé le lit 16, il passe de préférence par un séparateur de gouttes 17 afin de réduire la proportion de liquide entraîné par la phase gazeuse sortant du lit 16. Le séparateur 17 peut comprendre p.ex. des séparateurs à lamelles ou à tissus.The gaseous effluent 1 which enters the washer 9 can be brought into contact with the acid solution in co- or counter-current. In order to intensify or activate the contact and the exchange between the liquid and gaseous phase, it is possible to arrange in the washing column 9 a packing section 16 of any type known per se and comprising for example a bed , Berl, Raschig, Pall or other rings as filling material. Alternatively, columns of the spray type, trays, domes or any other contactor can be used, such as a liquid jet ejector, etc. After the gas has passed through the bed 16, it preferably passes through a droplet separator 17 in order to reduce the proportion of liquid entrained by the gas phase leaving the bed 16. The separator 17 can comprise, for example, lamellar separators or tissue.

La solution acide contenant l'urée est amené à une température de 30° à 60°C par une conduite 4 à des moyens de distribution 13 et pulvérisée, arrosée ou déversée par débordement ou par tout autre moyen sur le gaz de façon à assurer un bon contact ce qui produit une très intense absorption des vapeurs nitreuses. L'acide présente en général une concentration relativement forte comprise entre 5 et 15% en poids, p.ex. entre 5 et 10% en poids et peut comprendre soit un acide organique, soit de l'acide nitrique, chlorhydrique ou autre à condition que le pH soit compris entre 0 et 1,5, p.ex. entre 0,5 et 1. on ajoute à cette solution 50 à 500 g/l d'urée, soit sous forme solide, soit sous forme liquide.The acid solution containing the urea is brought to a temperature of 30 ° to 60 ° C by a line 4 to distribution means 13 and sprayed, sprinkled or poured by overflow or by any other means onto the gas so as to ensure a good contact which produces a very intense absorption of nitrous vapors. The acid generally has a relatively high concentration of between 5 and 15% by weight, eg between 5 and 10% by weight and can comprise either an organic acid or nitric, hydrochloric or other acid provided let the pH be between 0 and 1.5, eg between 0.5 and 1. 50 to 500 g / l of urea are added to this solution, either in solid form or in liquid form.

Le liquide de lavage acide est circulée et recirculée en fait au moyen d'une pompe 14 à partir d'une cuve de conditionnement 2. L'adjonction d'urée peut se faire par l'entrée 6 et celui de l'eau par l'entrée 7.The acid washing liquid is circulated and in fact recirculated by means of a pump 14 from a conditioning tank 2. The addition of urea can be done by the inlet 6 and that of the water by the 'entry 7.

En effet, on peut assister à une évaporation d'eau par la saturation adiabatique des gaz à épurer. Il convient de contrôler périodiquement la concentration en urée du liquide 4 à circuler pour l'ajuster aux quantités indiquées ci-dessus. Après le démarrage, il est inutile, en principe d'ajouter de l'acide dans ce premier étage. Un agitateur 12 peut être prévu pour assurer une bonne homogénéité de la solution et un système de réchauffage 11 permet de maintenir en température désirée le volume liquide.Indeed, one can witness an evaporation of water by the adiabatic saturation of the gases to be purified. The urea concentration of the liquid 4 to be circulated should be periodically checked to adjust it to the quantities indicated above. After start-up, it is useless, in principle to add acid in this first stage. An agitator 12 can be provided to ensure good homogeneity of the solution and a heating system 11 makes it possible to maintain the liquid volume at the desired temperature.

Le gaz ayant passé la première opération de lavage peut contenir encore des gouttes très fines acides qui doivent être neutralisées et éliminées avant que l'effluent gazeux s'échappe dans l'air. Le courant de gaz sortant du laveur 9 est donc transféré ensuite par la conduite 19 vers l'entrée 20 d'un second appareil de lavage et de neutralisation 10 qui peut être réalisé et fonctionner selon le même principe que le premier laveur 9. Le contact devra donc s'effectuer ici entre l'effluent gazeux et le liquide de lavage 5 qui est une solution alcaline dont le pH se situe entre 11 et 14, de préférence entre 12 et 14. L'agent alcalin peut être l'hydroxyde de sodium. Pour un pH de 13 à 14, la concentration s'élève de 5 X à 15% en poids. Comme pour ce qui a été décrit ci-dessus pour l'opération de lavage acide dans la colonne 9, le gaz est mis en contact dans le laveur 10 à co- ou contre-courant avec le liquide alcalin qui est amené par la conduite 5 et distribué par des moyens de distribution (pulvérisation, arrosage, déversement, jet de liquide ou autre). Le contact gaz/liquide peut également être intensifié en faisant passer le gaz à travers une section de garnissage 29 aménagée dans le laveur 10. Ce garnissage peut alors comprendre un lit d'anneaux de remplissage ou tout autre moyen permettant une augmentation considérable de la surface de contact gaz/liquide tel que décrit ci-avant pour la section 16. Le gaz lavé passe ensuite de préférence à travers un séparateur de gouttes 30, p.ex. un séparateur à lamelles ou à tissus avant d'être émis à l'atmosphère par la sortie 3. Il est à noter que le lavage alcalin ne sert pas seulement à neutraliser des restants d'acide, mais aussi à éliminer surtout des restants d'oxydes d'azote encore'présents dans le gaz entrant au piquage d'entrée 20.The gas having passed the first washing operation may still contain very fine acid drops which must be neutralized and eliminated before the gaseous effluent escapes into the air. The gas stream leaving the washer 9 is then transferred via line 19 to the inlet 20 of a second washing and neutralization device 10 which can be produced and operate on the same principle as the first washer 9. Contact must therefore be carried out here between the gaseous effluent and the washing liquid 5 which is an alkaline solution whose pH is between 11 and 14, preferably between 12 and 14. The alkaline agent can be sodium hydroxide . For a pH of 13 to 14, the concentration rises from 5 X to 15% by weight. As for what was described above for the washing operation acid in column 9, the gas is brought into contact in the washer 10 in co- or counter-current with the alkaline liquid which is brought by line 5 and distributed by distribution means (spraying, sprinkling, spillage, jet of liquid or other). The gas / liquid contact can also be intensified by passing the gas through a packing section 29 arranged in the washer 10. This packing can then comprise a bed of filling rings or any other means allowing a considerable increase in the surface. of gas / liquid contact as described above for section 16. The washed gas then preferably passes through a droplet separator 30, eg a flap or tissue separator before being emitted to the atmosphere via outlet 3. It should be noted that the alkaline washing is not only used to neutralize acid residues, but also to remove above all nitrogen oxide residues still present in the gas entering the nozzle. entry 20.

En fin d'opération on dispose donc de deux bains liquides résiduaires 2 et 8, l'un étant acide, l'autre alcalin et qui donc ne peuvent pas être rejetés comme tel. Dans certains cas il peut alors être utile de mélanger ces deux solutions résiduaires en vue d'une opération périodique ou finale de neutralisation mutuelle. La solution acide est alors pompé par la conduite 21 et la solution alcaline par la conduite 22 dans un récipient 18 où ils sont mélangés. Ce traitement additionnel de neutralisation fait donc appel à une technique simple et peu onéreuse, et le traitement global résulte alors dans une opération ne laissant pas de déchets nuisibles. Quand le mélange dans le récipient 18 contient encore des nitrites, ceux-ci devraient être transformés en nitrates par oxydation. Un agent oxydant est alors ajouté au récipient 18 par des moyens d'introduction 31.At the end of the operation, there are therefore two residual liquid baths 2 and 8, one being acidic, the other alkaline and which therefore cannot be rejected as such. In some cases it may then be useful to mix these two residual solutions for a periodic or final mutual neutralization operation. The acid solution is then pumped through line 21 and the alkaline solution through line 22 into a container 18 where they are mixed. This additional neutralization treatment therefore calls for a simple and inexpensive technique, and the overall treatment then results in an operation that does not leave harmful waste. When the mixture in container 18 still contains nitrites, these should be converted to nitrates by oxidation. An oxidizing agent is then added to the container 18 by introduction means 31.

Exemple 1Example 1

Un effluent gazeux contenant 120 ppm d'oxydes d'azote était traité avec le système de lavage acide et alcalin en cascade comme décrit ci-dessus. Le débit du courant de gaz 1 à l'entrée du laveur 9 était de 8000 m3/h. La solution acide/urée initiale présentait un pH de 0,75 et contenait 300 g/l d'urée. Sa température à l'endroit du pulvérisation 13 était de 30°C. Le débit de circulation de la solution acide/urée était de 35.000 1/h. Le lit d'anneaux de remplissage 16 permettait un temps de séjour de 2,5 à 3 sec. A la sortie 19 du laveur acide 9 la concentration en oxydes d'azote était déjà tombé à 18 ppm ce qui correspond à un rendement de 85 %. Le laveur alcalin 10 comprenait un lit 29 d'anneaux de remplissage identique au lit 16. La solution d'hydroxyde de sodium présentait un pH de 13 et elle était débitée par le pulvérisateur 28 à 35.000 1/h. A la sortie 3 du laveur alcalin 10 la concentration en oxydes d'azote avait encore diminuée au niveau de 7 ppm. L'efficacité du lavage alcalin s'élève donc à environ 60% et le rendement total du système global s'élève à 95 %.A gaseous effluent containing 120 ppm of nitrogen oxides was treated with the acid and alkaline cascade washing system as described above. The flow rate of the gas stream 1 at the inlet of the washer 9 was 8000 m 3 / h. The initial acid / urea solution had a pH of 0.75 and contained 300 g / l of urea. Its temperature at the place of spraying 13 was 30 ° C. The circulation rate of the acid / urea solution was 35,000 l / h. The bed of filling rings 16 allowed a residence time of 2.5 to 3 sec. At the outlet 19 of the acid scrubber 9, the concentration of nitrogen oxides had already fallen to 18 ppm, which corresponds to a yield of 85%. The alkaline washer 10 included a bed 29 of filling rings identical to bed 16. The sodium hydroxide solution had a pH of 13 and it was delivered by the sprayer 28 at 35,000 l / h. At the outlet 3 of the alkaline washer 10 the concentration of nitrogen oxides had further decreased to the level of 7 ppm. The efficiency of the alkaline washing therefore amounts to approximately 60% and the total yield of the overall system amounts to 95%.

Exemple 2Example 2

L'effluent gazeux à épurer était chargé de 6000 à 7000 ppm d'oxydes d'azote. Il était traité dans les mêmes conditions que celui de l'exemple 1. A la sortie 19 de la tour de lavage acide la concentration avait diminuée jusqu'à 900 - 1000 ppm ce qui indique encore un rendement d'environ 85 %. A la sortie du laveur alcalin 10, la concentration en oxydes d'azote avait tombée jusqu'à 500 ppm ce qui revient à un rendement d'environ 47 %. L'efficacité globale s'élève donc ici à environ 92 %.The gaseous effluent to be purified was charged with 6000 to 7000 ppm of nitrogen oxides. It was treated under the same conditions as that of Example 1. At the outlet 19 of the acid washing tower, the concentration had decreased to 900-1000 ppm, which again indicates a yield of approximately 85%. At the outlet of the alkaline washer 10, the concentration of nitrogen oxides had dropped to 500 ppm, which amounts to a yield of approximately 47%. The overall efficiency here therefore amounts to around 92%.

Claims (13)

1. Procédé de traitement d'effluents gazeux contenant des oxydes d'azote comprenant une opération de traitement des effluents gazeux par un liquide de lavage acide contenant de l'urée à une température entre 30* et 60°C caractérisé en ce qu'il comprend une seconde opération de traitement des effluents gazeux provenant dudit lavage acide par un second liquide de lavage alcalin.1. Process for the treatment of gaseous effluents containing nitrogen oxides comprising an operation of treatment of gaseous effluents with an acidic washing liquid containing urea at a temperature between 30 * and 60 ° C, characterized in that it comprises a second operation for treating the gaseous effluents from said acid wash with a second alkaline wash liquid. 2. Procédé selon la revendication 1 caractérisé en ce que le liquide de lavage acide comprend environ de 50 à 500 g d'urée par litre.2. Method according to claim 1 characterized in that the acid washing liquid comprises about 50 to 500 g of urea per liter. 3. Procédé selon la revendication 2 caractérisé en ce que le liquide de lavage acide a un pH entre 0 et 1,5.3. Method according to claim 2 characterized in that the acid washing liquid has a pH between 0 and 1.5. 4. Procédé selon la revendication 3 caractérisé en ce que le liquide de lavage a un pH entre 0,5 et 1.4. Method according to claim 3 characterized in that the washing liquid has a pH between 0.5 and 1. 5. Procédé selon l'une ou l'autre des revendications 1 à 3 caractérisé en ce que le liquide de lavage alcalin a un pH de 11 à 14.5. Method according to either of claims 1 to 3 characterized in that the alkaline washing liquid has a pH of 11 to 14. 6. Procédé selon l'une ou l'autre des revendications précédentes caractérisé en ce que les liquides résiduaires acides et alcalins sont mélangés en vue d'un traitement de neutralisation.6. Method according to either of the preceding claims, characterized in that the acid and alkaline waste liquids are mixed for the purpose of a neutralization treatment. 7. Procédé selon la revendication 6, caractérisé en ce que l'on ajoute un agent oxydant au mélange pour transformer les nitrites présents en nitrates.7. Method according to claim 6, characterized in that an oxidizing agent is added to the mixture to transform the nitrites present into nitrates. 8. Dispositif de traitement de effluents gazeux contenant des oxydes d'azote comprenant un appareil laveur (9) pourvu d'une entrée (1) pour l'effluent gazeux, des moyens d'alimentation (4) et de distribution (13) d'un liquide de lavage acide contenant de l'urée, d'une cuve (2) de collection du liquide de lavage sortant de l'appareil (9) et d'une conduite (19) d'évacuation de l'effluent gazeux caractérisé en ce que la conduite (19) est reliée à une entrée (20) d'un second appareil laveur (10) qui est pourvu d'un moyen d'alimentation (5) et de distribution (28) d'un liquide de lavage alcalin, d'une sortie (3) pour l'évacuation de l'effluent gazeux épuré et d'une cuve (8) de collection de ce liquide de lavage sortant de cet appareil (10), ladite cuve (8) étant pourvue de moyens (23, 25, 26, 27) de conditionnement et de moyens (24) de recirculation du liquide alcalin.8. Device for treating gaseous effluents containing nitrogen oxides comprising a washing device (9) provided with an inlet (1) for the gaseous effluent, means of supply (4) and distribution (13) d '' an acidic washing liquid containing urea, a tank (2) for collecting the washing liquid leaving the device (9) and a pipe (19) for evacuating the gaseous effluent characterized in that the line (19) is connected to an inlet (20) of a second washing device (10) which is provided with means for supplying (5) and distributing (28) a washing liquid alkaline, an outlet (3) for the evacuation of the purified gaseous effluent and a tank (8) for collecting this washing liquid leaving this device (10), said tank (8) being provided with means (23, 25, 26, 27) for conditioning and means (24) for recirculating the alkaline liquid. 9. Dispositif selon la revendication 8 caractérisé en ce que le laveur (10) comprend en outre des moyens (29) d'activation de la mise-en-contact des phases liquides et gazeuses.9. Device according to claim 8 characterized in that the washer (10) further comprises means (29) for activating the bringing into contact of the liquid and gaseous phases. 10. Dispositif selon la revendication 8 ou 9 caractérisé en ce que le laveur (10) comprend en outre en aval des moyens de distribution (28) et des moyens d'activation de mise en contact (29) des moyens (30) de séparation de gouttes.10. Device according to claim 8 or 9 characterized in that the washer (10) further comprises downstream distribution means (28) and contacting activation means (29) means (30) of separation drops. 11. Dispositif selon l'une ou l'autre des revendications précédentes caractérisé en ce qu'il comprend en outre un récipient de neutralisation (18) pourvue des moyens de connection (21) avec la cuve (2) du liquide de lavage acide d'une part et des moyens de connections (22) avec la cuve (8) du liquide de lavage alcalin d'autre part.11. Device according to either of the preceding claims, characterized in that it further comprises a neutralization container (18) provided with connection means (21) with the tank (2) of the acid washing liquid d 'on the one hand and connection means (22) with the tank (8) of the alkaline washing liquid on the other hand. 11. Dispositif selon l'une ou l'autre des revendications précédentes caractérisé en ce qu'il comprend en outre un récipient de neutralisation (18) pourvue des moyens de connection (21) avec la cuve (2) du liquide de lavage acide d'une part et des moyens de connections (22) avec la cuve (8) du liquide de lavage alcalin d'autre part.11. Device according to either of the preceding claims, characterized in that it further comprises a neutralization container (18) provided with connection means (21) with the tank (2) of the acid washing liquid d 'on the one hand and connection means (22) with the tank (8) of the alkaline washing liquid on the other hand. 13. Dispositif selon la revendication 11 caractérisé en ce que le récipient (18) est pourvu en outre des moyens d'introduction (31) d'un agent oxydant.13. Device according to claim 11 characterized in that the container (18) is further provided with introduction means (31) of an oxidizing agent.
EP86201521A 1985-09-04 1986-09-04 Process and apparatus for eliminating nitrogen oxides from a gaseous exhaust stream in the humid way Withdrawn EP0217437A1 (en)

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FR8513146 1985-09-04
FR8513146A FR2586586B1 (en) 1985-09-04 1985-09-04 REMOVAL OF NITROGEN OXIDES FROM A WET GAS EFFLUENT.

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EP0426215A1 (en) * 1989-10-06 1991-05-08 Leonardus Mathijs Marie Nevels Method for purifying flue gases
FR2990629A1 (en) * 2012-05-15 2013-11-22 Lab Sa GLOBAL METHOD FOR WET CLEANING SMOKE

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EP0426215A1 (en) * 1989-10-06 1991-05-08 Leonardus Mathijs Marie Nevels Method for purifying flue gases
US5186916A (en) * 1989-10-06 1993-02-16 Nevels Leonardus M M Method for purifying flue-gasses
FR2990629A1 (en) * 2012-05-15 2013-11-22 Lab Sa GLOBAL METHOD FOR WET CLEANING SMOKE

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FR2586586B1 (en) 1988-10-28

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